1. FIELD OF THE INVENTION
The invention relates to computer monitors particularly with respect to enhancing the character display capacity thereof.
2. DESCRIPTION OF THE PRIOR ART
Present day computer monitors display alphanumeric text by utilizing adjacently disposed character matrices of pixels (picture elements) for displaying the characters. An alphanumeric character is displayed in a pixel matrix by selectively illuminating the pixels of the matrix. Intercharacter spacing is effected by reserving one or more rows and columns of pixels at the edges of the matrix for the space. The number of pixels that can be displayed on a line is limited by the bandwidth of the video driving circuitry and thus the number of characters that can be displayed in a character row of the monitor is similarly limited by the video bandwidth. In order to maximize the number of characters on a line, the frequency of the pixel clock is set for operation at the high frequency band edge of the video amplifier.
If the frequency of the pixel clock is increased somewhat so that operation occurs slightly beyond the high frequency end of the video amplifier bandwidth in order to increase the monitor capacity, a phenomemon denoted as pixel or dot smearing occurs. Since the slew rate of the video amplifier is inadequate to rapidly turn on and off at this increased frequency, the video amplifier does not completely turn off the video gun at the end of a pixel before the start of the next pixel. An adequate black level between pixels is not achieved resulting in inadequate contrast between the dots. Adjacent pixels tend to merge into one another. Because of this phenomemon, elements of the characters run into each other and adjacent characters run into each other severely degrading the legibility of the display.
When utilizing a system that operates just beyond the band edge of the video amplifier, additional characters may be displayed on a line by reducing the width of the character matrix and utilizing a smaller intercharacter spacing. This approach, coupled with the pixel smearing effect further exacerbates display illegibility.
A prior art approach to increasing the number of characters on the line is to increase the bandwidth of the video channel and hence increase the slew rate. The increased slew rate permits the beam to be turned on and off at a high rate thus generating an increased number of pixels on the line with adequate black level therebetween. Increased bandwidth, however, significantly increases the cost of the monitor.